1. Field of the Invention
The present invention generally relates to an optical transmission system that can be applicable to a long distance, high capacity transmission.
The present invention also relates to optical transmission devices, such as a transmitter and a receiver for the optical transmission system.
Such types of high capacity transmission systems using optical signals have been developed and designed so as to be adapted to multimedia applications. Many TDM (Time Division Multiplexing) transmission systems or WDM (Wavelength Division Multiplexing) transmission systems have been known. Typically, those systems have been intended to efficiently make use of a transmission line. In these high capacity transmission systems, it is particularly demanded that a reliable transmission can be achieved.
Therefore, the present invention relates to, in particular, the optical transmission system that has their transmission reliability been improved and the optical transmission device used in this system.
2. Description of the Related Art
A conventional wavelength-multiplexing transmission system includes an optical transmitter 201, an optical transmission line 203 and an optical receiver 202, as schematically shown in FIG. 1, and the system is in conformity with SDH (Synchronous Digital Hierarchy) that is a set of international, digital transmission standards. The optical transmitter 201 has, for each of k channels CHi (i=1, . . . , k), individually an SOH (Section Over Head) inserting unit 204 for inserting an SOH, an electrical-optical converter (OS) 205 and a wavelength-multiplexer 206. The optical receiver 202 also has, for each of the k channels, individually a wavelength-demultiplexer 207, an optical-electrical converter (OR) 208 and an SOH terminating unit 209.
The SOH inserting unit 204 at the optical transmitter 201 inserts the SOH into an electrical signal for one of the corresponding channels CHi. Each electrical signal for the every channel is then provided to the optical-electrical converter 205 and converted to an optical signal with a wavelength xcexi corresponding to the channel CHi. The optical signals having the wavelength of xcexi are multiplexed by the wavelength-multiplexer 206 and resulting wavelength-multiplexed signals are transmitted to the optical transmission line 203.
The wavelength demultiplexer 207 at the optical receiver 202 separates the multiplexed signals received from the optical transmitter 201 through the optical transmission line 203 into the signals corresponding to the wavelengths xcex1 to xcexk, respectively. These optical signals having the wavelength of xcex1 to xcexk, respectively, are converted to corresponding electrical signals by the optical-electrical converter 208, and then the SOH of the electrical signals is terminated by the SOH terminating unit 209. The electrical signals having their SOH terminated are transmitted to a further stage (not shown in FIG. 1) on an each (i.e., individual) channel basis. Thus, the data comprising the electrical signals for each of the channels CH1 to CHk can be transmitted from the optical transmitter 201 to the optical receiver 202 over the signal optical transmission line 203.
Several error correction techniques have been also proposed in order to improve a transmission quality by correcting transmission errors involved in the transmitted data. For example, one of the known techniques, also called an xe2x80x9cFEC (Forward Error Correction)xe2x80x9d method, consists in generating and adding an error correction bit to the data representing one frame or the data of a predetermined length and performing the error correction at a receiver side.
Adding a parity bit to the transmitted data is also a common technique used for determining a presence/absence of the transmission error within the transmitted data. In this case, the SOH may be also provided with error monitoring bits, named B1 and B2.
The earlier described error correction techniques consist in, for every frame or every block of the transmission data, generating an error correction bit and adding it to each frame or block. Therefore, in contrast with a transmission system without correcting transmission errors, the conventional transmission system provided with the error correction technique has to increase a transmission rate, because a number of bits to be transmitted are increased. Alternatively, if the transmission rate is set to a predetermined value, the transmission system should reduce an amount of the transmission data so that the error correction bit can be transmitted together with the transmission data within the predetermined transmission rata.
Furthermore, in some of the conventional transmission systems, erroneous bits included in the transmission data cannot be corrected when parity bits are contained in the data. One solution for improving a capability of correcting the erroneous bits in the data is to increase the number of the error correction bits added to the transmission data. However, this solution may be not practical, because a considerably high transmission rate is required for increasing the number of error correcting redundant bits to be added to the transmission data.
Another possible solution is to insert the error correction bits into reserved bits within the SOH. The reserved bits means that those bits are reserved for a variety of future applications. In this case, since a lot of redundant bits are to be inserted into some particular locations in the SOH, a problem may occur that a size of a circuit comprising a transmission device, such as the transmitter 201 and the receiver 202, is enlarged. This solution has a further drawback in that the error correction bits, which have been already assigned to the reserved bits, cannot be made use of, if the reserved bits are decided to be used for one of the future applications.
Accordingly, an object of the present invention is to provide an optical transmission system for allowing a high capacity and high quality transmission and which can be easily and simply manufactured or implemented.
Another object of the present invention is to provide an optical transmitter and an optical receiver suitable for used in the optical transmission system according to the present invention.
The object of the present invention is achieved by an optical transmission system which is operable to form a set of k data by aligning each of phases from k channels in phase, generate and add a set of (nxe2x88x92k) error correction bits to the set of k data so as to produce n data in total, convert the n data into different signals having different wavelengths xcex1 to xcexn, respectively, by an electrical-optical converter, multiplex these signals by an wavelength multiplexer, and send the multiplexed signals to an optical transmission line.
The inventive optical transmission system further operable to receive the multiplexed signals through the optical transmission line, separate the received multiplexed signals into signals having different wavelengths xcex1 to xcexn, respectively, by a wavelength demultiplexer, converts the signals having the different wavelengths xcex1 to xcexn respectively, to electrical signals by an optical-electrical converter, and correct errors within the k data by means of the (nxe2x88x92k) error correction bits contained in the n data.
In the optical transmission system according to the present invention, the k data concurrently transmitted are added to in parallel by the (nxe2x88x92k) error correction bits. Then, the k data being added to by the error correction bits are converted to optical signals having the different wavelengths xcex1 to xcexn, respectively, so as to be transmitted as the wavelength-multiplexed optical signals. This allows the optical transmission system to correct the errors at a receiver and transmit data with the high quality without increasing the transmission rate. In addition, since the error correction bits are generated for the k data at the same timing, an error correction decoding process has to be performed at the receiver. To do this, a frame synchronous byte may be added to each of the k data, each data containing the error correction bits.
The object of the present invention can be achieved by a further optical transmission system which is operable to form a set of k data by aligning data from k channel CHi to CHk with each other, add an SOH containing error monitoring bytes B1 and B2 to the set of k data, generate and add a parity bit to the set of the k data so as to form a sequence of (k+1) data, convert the sequence of the (k+1) data to optical signals having different wavelengths, wavelength-multiplex the optical signals and send the multiplexed signals to an optical transmission line. At the receiver, after receiving the multiplexed signals through the optical transmission line, the received multiplexed signals are separated into signals corresponding to wavelengths, respectively, and then the separated signals are converted to electrical signals. Subsequently, a parity check is performed on the basis of a sequence of (k+1) data from the electrical signals, and another parity check corresponding to channels CH1 to CHk is carried out by means of an error check byte within the SOH. In this case, depending on results of the parity checks, a position of the error bit is located and the error bit may be corrected. Thus, the error correction can be achieved solely by additionally assigning the parity bit corresponding to a vertical parity to the data.
The object of the present invention can be achieved by a still further optical transmission system for serially transmitting data such as TDM (Time-Division Multiplexing) transmission data. The optical transmission system is operable to generate and add a set of (nxe2x88x92k) error correction bits to k bits of the transmission data, convert the k bits of the transmission data and the (nxe2x88x92k) error correction bits to different optical signals having different wavelengths, and the optical signals are multiplexed so as to be transmitted as wavelength-multiplexed signals through an optical transmission line. The optical transmission system further receives the wavelength-multiplexed signals through the optical transmission line, separates the received multiplexed signals into different signals having corresponding wavelengths, and perform error correction decoding process for the n bits corresponding to the transmission data by means of the (nxe2x88x92k) error correction bits. That is to say, in this case, for a serial data of the k bits, the (nxe2x88x92k) error correction bits are converted into the signals having the different wavelengths such that the serial data of k bits can be transmitted in parallel.
The object of the present invention can be achieved by a still further optical transmission system for transmitting data through k channels. The optical transmission system, at a transmitter, generates and adds (nxe2x88x92k) error correction bits to k data present at the same timing so as to form a sequence of n data, multiplexes the sequence of the n data, converts the multiplexed data to optical signals and sends the optical signals to an optical line. The optical transmission system, at a receiver, receives the optical signals through the optical line, converts the received optical signals to electrical signals, separates the electrical signals into a sequence of n data and performs an error correction decoding on the k data from the sequence of the n data by means of the (nxe2x88x92k) error correction bits. In this case, if a plurality of reserved channels exist in the channels available in a TDM (Time Division Multiplexing) transmission, the reserved channels can be assigned to the error correction bits. Alternatively, if a number of the error correction bits assigned to busy channels is more than that of the reserved channels, an error correction coding is performed only on some significant channels from the busy channels. It leads to that the number of the error correction bits within an error correction code is limited by the number of the reserved channels and the data transmitted through the significant channels can be transmitted using the error correction coding.